Any warrior with even a little battle hardening will agree that the safest ground to defend is the high ground. And if there is no high ground to defend, make it yourself.
Stairs are a necessity once buildings grow beyond a certain size. They provide points of natural defence and easy access to higher floors and rooftops (which make excellent archery platforms). Their only weakness is the material upon which they are built - if the high ground is brought crashing down then it no longer serves a tactical purpose.

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